Fusion of Wildlife Tracking and Satellite Geomagnetic Data for the Study of Animal Migration Fernando Benitez-Paez1,2 , Vanessa Da Silva Brum-Bastos1 , Ciarán D
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Benitez-Paez et al. Movement Ecology (2021) 9:31 https://doi.org/10.1186/s40462-021-00268-4 METHODOLOGY ARTICLE Open Access Fusion of wildlife tracking and satellite geomagnetic data for the study of animal migration Fernando Benitez-Paez1,2 , Vanessa da Silva Brum-Bastos1 , Ciarán D. Beggan3 , Jed A. Long1,4 and Urška Demšar1* Abstract Background: Migratory animals use information from the Earth’s magnetic field on their journeys. Geomagnetic navigation has been observed across many taxa, but how animals use geomagnetic information to find their way is still relatively unknown. Most migration studies use a static representation of geomagnetic field and do not consider its temporal variation. However, short-term temporal perturbations may affect how animals respond - to understand this phenomenon, we need to obtain fine resolution accurate geomagnetic measurements at the location and time of the animal. Satellite geomagnetic measurements provide a potential to create such accurate measurements, yet have not been used yet for exploration of animal migration. Methods: We develop a new tool for data fusion of satellite geomagnetic data (from the European Space Agency’s Swarm constellation) with animal tracking data using a spatio-temporal interpolation approach. We assess accuracy of the fusion through a comparison with calibrated terrestrial measurements from the International Real-time Magnetic Observatory Network (INTERMAGNET). We fit a generalized linear model (GLM) to assess how the absolute error of annotated geomagnetic intensity varies with interpolation parameters and with the local geomagnetic disturbance. Results: We find that the average absolute error of intensity is − 21.6 nT (95% CI [− 22.26555, − 20.96664]), which is at the lower range of the intensity that animals can sense. The main predictor of error is the level of geomagnetic disturbance, given by the Kp index (indicating the presence of a geomagnetic storm). Since storm level disturbances are rare, this means that our tool is suitable for studies of animal geomagnetic navigation. Caution should be taken with data obtained during geomagnetically disturbed days due to rapid and localised changes of the field which may not be adequately captured. Conclusions: By using our new tool, ecologists will be able to, for the first time, access accurate real-time satellite geomagnetic data at the location and time of each tracked animal, without having to start new tracking studies with specialised magnetic sensors. This opens a new and exciting possibility for large multi-species studies that will search for general migratory responses to geomagnetic cues. The tool therefore has a potential to uncover new knowledge about geomagnetic navigation and help resolve long-standing debates. Keywords: Animal migration, Data fusion, Earth’s magnetic field, GPS tracking, Swarm satellite constellation * Correspondence: [email protected] 1School of Geography and Sustainable Development, Irvine Building, University of St Andrews, North Street, St Andrews KY16 9AL, Scotland, UK Full list of author information is available at the end of the article © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Benitez-Paez et al. Movement Ecology (2021) 9:31 Page 2 of 19 Background Migration studies now frequently use tracking data, Long-distance migratory navigation consists of two collected with in-situ locational devices (such as GPS parts, determining the direction of movement (through loggers) which record the location of animals during compass orientation) and geographic positioning, that is, their journeys. Tracking, combined with displacement, knowing where the animal is located at a specific time has become a common way of investigating a particular [1]. Both these mechanisms support the so-called true navigational cue (e.g. see [18] for an example of such an navigation, which is defined as finding the way to a far experiment for both geomagnetic and olfactory naviga- away unknown location using only cues available locally tion; some other examples include [20, 21]). Some stud- [2]. Compass orientation uses information from the Sun, ies have explored geomagnetic navigation by modelling the stars, the polarised light and the Earth’s magnetic potential migratory pathways based on real tracking data field [3, 4]. Positioning uses geomagnetism [3], olfactory and a static representation of the geomagnetic field [22]. cues [5, 6], and visual cues such as landmarks [7], while However, these fail to consider temporal variation in the another possible cue is natural and anthropogenic field, which may be problematic, as solar wind induced infrasound [1], although there are only a few studies on short-term variations of the geomagnetic field are this mechanism. It has been hypothesised that some ani- greater than the recorded magnetic sensitivity of migra- mals (birds, turtles, and fish) use sensory information tory animals. Neurophysiological experiments have from these cues to generate multifactorial internal maps shown that birds can sense changes in geomagnetic in- [7–9], although there is an on-going debate about the tensity between 50 and 200 nanoTesla (nT) [23, 24], and existence of such maps, as this is very difficult to con- behavioural experiments suggest sensitivity of 15-25 nT firm experimentally [2, 3]. [25]. Solar wind disturbances, however, can often reach One of the migratory strategies is geomagnetic navi- variations of over 1000 nT in polar latitudes within mi- gation [3], which uses information from the Earth’s nutes during geomagnetic storms [26]. That is, the field magnetic field for either compass orientation or geo- intensity changes across a very short period (seconds to graphic positioning or both. Various geomagnetic minutes) for over 1000 nT in the same location (not navigation mechanisms have been observed across across a spatial range, but in the same place). Migratory several taxa [3], from birds [1, 7], fish [9], sea turtles animals may therefore be impacted by such dynamic [8, 10], terrestrial [11, 12] and sea mammals [13, 14]. conditions. For example, looking specifically at birds, if In birds, for example, the strongest evidence for geo- they use the intensity value as a location marker, they magnetic navigation comes from studies that either may think they are suddenly somewhere else and could manipulate animal’s perceived magnetic position and try to compensate by changing their flight direction back observe their subsequent re-orientation towards mi- to their migratory corridor, as shown in virtual magnetic gratory destinations [15, 16] or those that surgically displacement studies [15–17, 27]. If the storm distur- manipulate animals’ organs that may help sense mag- bances are strong and come from many directions, this netic field, such as the trigeminal nerve. One study compensation could result in increased variation in [17] sections this nerve in Eurasian reed warblers and attempted flight directions. Alternatively, if they use dir- “virtually displaces” the birds using an artificial mag- ectional components of the field, such as inclination, netic field, then observes that the manipulated birds they may lose their compass sense and either change dir- are not able to correct their direction. Although see a ection or switch their navigation to other types of com- GPS study for the opposite finding in lesser black- passes that may be available at that particular location backed gulls [18]. In spite of decades of research, we and moment in time, e.g. a Sun or a star compass [4]. still do not fully understand how exactly animals use Other animals may react in different ways, depending on the information provided by the Earth’s magnetic field their particular manner of using the geomagnetic infor- toachievetruenavigation[1]. mation for navigation [3]. Indeed, geomagnetic storms Geomagnetic navigation has been studied with la- could be linked to large strandings of whales [13, 14], al- boratory experiments, which place animals in an arti- though this is not fully confirmed - see [28] for a coun- ficial magnetic field to study how the magnetic field ter argument. influences the direction of the onset of migration [2, Such effects would most likely be the highest during 7, 15, 17]. Such experiments provide precision and geomagnetic storms when the temporal variations of the control, but observed behaviour in such experiments field are the largest. To study how both long- and short- may differ from that observed in the wild [1]. A fur- term variation of the geomagnetic field affects migratory ther limitation is that these experiments focus on a navigation, we therefore need to obtain accurate values small number of individuals from one single species, of the geomagnetic field at the locations and times of which limits the generalisability of results across mul- animal passage. Satellite geomagnetic data, which pro- tiple species and taxa [19]. vide continuous global coverage, offer great potential for Benitez-Paez et al. Movement Ecology (2021) 9:31 Page 3 of 19 this purpose, but there is currently no tool in existence generated through external influences [34].